Prior art acoustic mufflers are generally of two types, friction type mufflers which place rigid barriers such as baffle plates with apertures into the path of the gas flow to break up and mix the sound waves, and absorption mufflers which absorb the sound waves in an acoustic damping material.
The friction type muffler is used most frequently, particularly on automobiles. This type of muffler typically has a casing with an inlet and outlet which can be positioned in a variety of locations, and a series of baffle plates there between to direct the gas flow in a circuitous route from inlet to outlet to cause mixing of the gas flow. Offset perforated inlet and outlet pipes may each extend the length of the casing to provide the circuitous route. Friction type mufflers are generally quite effective at reducing noise levels, but can also offer substantial resistance to gas flow because of the circuitous route followed by the exhaust gases passing through the muffler. Therefore, significant pressure is required to force the gases through the muffler. This additional pressure, referred to as back pressure, reduces the efficiency and power output of the source device being muffled.
The typical absorption type muffler has a casing with a pipe extending completely therethrough. A portion of the pipe inside the casing is perforated and the space between the pipe and casing is filled with sound absorbing fiberglass, ceramic fibers, or metallic wool mesh to absorb sound waves. By allowing the exhaust gases to pass directly through the muffler, the back pressure required to push the gas through the muffler is significantly reduced in comparison with friction type mufflers and more flow is obtained from the source device. However, the sound attenuation is often much less than that obtained with friction mufflers, making this type of muffler unacceptable in many applications.
Muffler acoustic efficiency is measured in decibels of noise attenuation (dba) versus gas flow in cubic feet per minute (CFM). When a pressure difference of 5 inches of water is imposed between the inlet and outlet, and using a common 2½ inch diameter muffler inlet and outlet, friction type mufflers have about 10-18 dba attenuation and 70-100 CFM flow. Absorption type straight through mufflers under those conditions have an attenuation of about 2-7 dba and 200 CFM flow.
There is a need in many applications for a muffler which has greater acoustic attenuation than the absorption type muffler, but with higher flow rates and less back pressure than the friction type mufflers.